JPH01268455A - Input voltage detecting circuit for switching power supply - Google Patents

Abstract

PURPOSE:To minimize power consumption by providing a voltage dividing resistor and a transistor so as to detect an input power and by giving a switching control circuit a signal designating as to whether oscillation is allowable or not. CONSTITUTION:A switching power supply is composed of a primary side rectifier circuit 2 rectifying AC input 1, a primary side smoothing capacitor C1, a transformer 3, a switching FET 4 and its switching control circuit 5, a secondary side rectifier circuit 6, and a voltage detecting circuit 9. In this case, said voltage detecting circuit 9 divides a voltage VC1 across the ends of said smoothing capacitor C1 on the input side by resistors R1-R3 in order to detect the voltage of said AC input 1, and turns ON or OFF a transistor(Tr) 1 by a detected voltage. Then, when said Tr 1 is turned ON, the signal designating that oscillation is not allowable is given to the C terminal of said control circuit 5, and when said Tr 1 is turned OFF, the signal designating that oscillation is allowable is given to said CS terminal. The voltage detecting circuit 9 is further provided with Tr 2 to short-circuit the resistance R3 in the ON state. Thus, the Trs 1-2 are OFF and a power consumption is minimized at the time of normal oscillation.

Description

[Detailed description of the invention] [Industrial application field]

The present invention relates to a voltage detection circuit for an input power supply of a switching power supply.

[Conventional technology]

In a switching power supply, if the voltage of the input power supply falls below the rated range, switching control may not be performed normally, and a large current may flow through the circuit, causing the circuit to burn out. Therefore, a method is known in which a voltage detection circuit for the input power source is provided to control the switching operation.

[Problem to be solved by the invention]

However, in the past, this input voltage detection circuit sometimes consumed power and reduced the conversion efficiency of the switching power supply. Therefore, an object of this invention is to minimize the power consumed by the voltage detection circuit when the input power supply voltage is within the rated range in a switching power supply circuit, and also to minimize the power consumed by the voltage detection circuit when the input power supply voltage is turned on and the switching start voltage when the input power supply is turned on and the switching when the power supply is turned off. The purpose is to configure a hysteresis circuit with a difference in stop voltage and to perform stable voltage detection of a human power source.

[Means to solve the problem]

In order to solve the above problems, the circuit of the present invention repeatedly opens and closes the input DC voltage (capacitor voltage VC1, etc.) via the first transistor (switching FET 4, etc.) to connect the primary winding of the transformer (3, etc.). (31, etc.), thereby rectifying the voltage generated in the secondary winding (32, etc.) of the transformer to obtain a new DC voltage (DC ratio cuff, etc.), in which the input DC voltage is Voltage dividing resistors (resistors R1 to R3, etc.) that divide the voltage to obtain a comparison voltage (detection voltage VA, etc.); Generates and stops an oscillation voltage that drives the first transistor according to a second transistor (such as Tri) that turns off and on, and (signals of) the transistor that turns off and on (applied to terminal C3, etc.) A transistor connected in parallel with a switching control circuit (such as 5) that causes the voltage to be set, and a partial resistor (such as R3) included in the resistance side (R2, R3, etc.) of the voltage dividing resistor on the side from which the comparison voltage is obtained. , turned on depending on whether the second transistor is turned on or off, respectively;
and a third transistor (such as Tr2) that is turned off. shall be equipped with the following.

[For use]

To detect the voltage of the input power supply of a switching power supply,
Resistors R1 to R3 divide the voltage VCI across the smoothing capacitor on the input side and set the detection voltage. The detection voltage VA divided by these resistors causes O
A transistor Tri that is turned off is provided, and when the transistor Tri is turned on, the C of the switching control circuit 5 is
Give a signal to the S terminal that disables oscillation and enables oscillation by turning it off. Furthermore, this transistor Tri stops supplying the base current and turns ON. A transistor Tr2 that is turned off is provided, and this transistor Tr2 is connected to the voltage V across the smoothing capacitor on the input side.
A Schmitt circuit is constructed in which a part of the resistor R3 that divides CI is short-circuited in the ON state, and the voltage division ratio is changed to provide a difference ΔVH between the oscillation start voltage and the oscillation stop voltage. With this configuration, these two transistors Tri and Tr2 are in an OFF state in a steady oscillation state, so the power consumed by this detection circuit is minimized.

【Example】

FIG. 1 is a circuit diagram showing a configuration as an embodiment of the present invention, and (A) and (B) in the figure show the transformer 3 when the switching power supply is configured as a forward type and a flyback type, respectively. A secondary circuit is shown. Note that the forward type here refers to the primary winding of transformer 3, 13
A flyback type refers to a method in which a DC output voltage is obtained by rectifying the voltage induced in the secondary winding 32 of the transformer 3 when an input DC voltage is applied to the transformer 3. A method of obtaining a DC output voltage by rectifying the voltage induced in the secondary winding 32 of the transformer 3 when the input DC voltage applied to the transformer 3 is cut off. In the figure, 1 is an AC input, 2 is a primary side rectifier circuit as a diode bridge that rectifies this AC human power 1, and C
I is a primary side smoothing capacitor that smoothes this rectified voltage. Here, the voltage across the capacitor C1 (also referred to as capacitor voltage) is defined as VCI. 3 is trance, 4
is the output DC voltage (capacitor voltage) of the primary rectifier circuit 2
A switching FET 5 which switches the VCI on and off and applies it to the primary winding 31 of the transformer 3 is a switching control circuit that controls the opening and closing of the FET 4. In this control circuit 5, VDD is a driving power supply supplied through an appropriate means not shown, GND is a ground potential, and C
8 is a terminal that controls switching oscillation sound and disable, and C
3 is H level (#VDD), oscillation is not possible, L level (!=
i0■) enables oscillation. 6 (6162) are forward type (same figure (A)
). This is a secondary rectifier circuit in the flyback type ((B) in the same figure), which rectifies and smoothes the voltage generated by the secondary winding 32 of the transformer 3 and supplies it to the load as a DC cuff. The voltage detection circuit 9 detects the voltage VCI across the negative side smoothing capacitor C1, and transmits the detected result to the CS terminal of the switching control circuit 5. Next, the operation of the voltage detection circuit 9 will be explained. (i) Immediately after power-on (oscillation stopped state): Immediately after power-on,
Since capacitor C1 is not sufficiently charged yet, capacitor voltage VCI is low. In this case, VCI is set to R1, R.
2. The detection voltage VA divided by the resistor R3 is VDD-VA≧VIBE (VIBE is the base-emitter voltage at which the transistor Tri is turned on) (1) Since it satisfies 2, the base current in Trl is flow T
rl becomes ON state. Furthermore, by turning on Tri, a base current is supplied to transistor Tr2, and Tr2 is also turned on.
The state is N, and the voltage at the CS terminal of the control circuit 5 is approximately VD.
Equal to D, the oscillation is stopped and the FET 4 is in the OFF state. (ii) Input DC voltage (capacitor voltage) After establishing VCI: After the power is turned on, capacitor CI is gradually charged and the detection voltage VA, which is the divided capacitor voltage VCI, is V DD -V
A < V IBIE -1---
-(2) When the above formula (2) is satisfied, the transistor Tr
i turns off. When this Tri is turned off, the voltage at the terminal C3 becomes L level (!=i0V) and oscillation becomes possible. At this time, since Tri is in the OFF state, the base current is not supplied to the transistor Tr2, and the transistor Tr2 is turned OFF. Therefore, during steady oscillation, the two transistors Trl and Tr2 are in the OFF state, so power consumption is minimized. Regarding the voltage VCI across the capacitor C1, which is the limit at which oscillation is possible, since the transistor Tr2 is ON in the state (i) above, the capacitor voltage C1 is divided by the resistors R1 and R2, and the detection voltage VA is Below (3)
It is given by Eq. R1+R2 (However, V 2CEζ0 is assumed.) Therefore, the capacitor voltage VCI is calculated using equation (2), I VCl2(1+-)(VDD-VIBE) -・-(
4) makes it possible to oscillate. (iii) When the power supply is cut off: During steady oscillation, when the input power supply is cut off, the voltage across the capacitor C1, ■C1, gradually drops. At this time, since the transistor Tr2 is initially in the OFF state as mentioned in the previous comment, the capacitor voltage ■C1 is reduced by the resistors R1, R1, R
The pressure is divided by 3. Therefore, the detection voltage VA is expressed by the following equation (5). Next, when the capacitor voltage falls and the detection voltage VA satisfies the following equation (6), the transistor Tri is turned on.
The L and CS terminals become H level (!=iVDD) and oscillation stops. VDD-VA≧VIBE-m---・
- (6) (5), From (6), the voltage across the capacitor CI at which oscillation stops ■C1 is given by the following equation (7) I
? I The difference ΔVH between the oscillation start voltage in equation (4) and the oscillation stop voltage in (7) 2 is expressed by equation (8) below from equations (4) and (7). I? 2(R2+R3) In other words, this difference voltage Δ■11 corresponds to the hysteresis voltage of the Schmitt circuit, causing the switching control circuit 5 to unstably reciprocate between the two states of oscillation start and oscillation stop ( hunting).

【Effect of the invention】

According to the present invention, the input DC voltage ■C1 is repeatedly opened and closed via the first transistor FET, and the primary winding 3 of the transformer 3 is
1 and thereby rectify the voltage generated in the secondary winding 32 of the transformer to obtain a new DC voltage 7. At each switching power supply number, the input DC voltage is divided and a comparison voltage (detection voltage ■ Voltage dividing resistors R1-R3 to obtain 8),
The comparison voltage VA is a predetermined voltage (VDD-VIBE)
a second transistor Tr2 that turns off and turns on depending on whether or not the voltage exceeds Tr2; and a switching control circuit that generates and stops an oscillation voltage that drives the first l transistor depending on whether the transistor is turned off or turned on. 5 and a transistor connected in parallel with a partial resistor R3 included in resistance sides R2 and R3 of the voltage dividing resistor on the side from which the comparison voltage is obtained,
Turns on depending on whether the second transistor is turned on or off. Since the third transistor Tr2 is turned off, it is possible to prevent the switching power supply circuit from collapsing due to abnormal oscillation when the input voltage is below the rated input range, and to prevent the switching power supply circuit from collapsing due to abnormal oscillation when the input voltage is below the rated input range. , it is possible to minimize the power consumption in the voltage detection circuit 9 and prevent the conversion efficiency of the switching power supply from decreasing. Further, since this voltage detection circuit 9 is configured as a Schmitt circuit, it is possible to stably obtain signals for starting and stopping oscillation.

[Brief explanation of the drawing] FIG. 1 is a circuit diagram as an embodiment of the present invention.

Claims (1)

[Claims] 1) The input DC voltage is repeatedly opened and closed via a first transistor and applied to the primary winding of the transformer, thereby rectifying the voltage generated in the secondary winding of the transformer and generating a new voltage. A switching power supply that obtains a direct current voltage, comprising: a voltage dividing resistor that divides the input direct current voltage to obtain a comparison voltage; and a second resistor that turns off and on depending on whether the comparison voltage exceeds a predetermined voltage. a switching control circuit that generates and stops an oscillation voltage that drives the first transistor depending on whether the transistor is turned off or on; and a third transistor connected in parallel with a partial resistor that is turned on and off in response to turning on and off of the second transistor. Voltage detection circuit.